Recently, with the progress of high sensitivity and small format of silver halide color negative photographic materials, the development of color negative photographic materials having high sensitivity and giving excellent image quality has been keenly desired.
For this purpose, the requirement for silver halide photographic emulsions has become more and more severe and a higher level of photographic performance such as high sensitivity, high contrast, excellent granularity, high sharpness, etc., has been required.
For such requirements, the technique of using tabular grain silver halide emulsions aiming at the improvement of sensitivity including color sensitization effect by sensitizing dyes, the improvement of the relation between sensitivity and granularity, the improvement of sharpness, and the improvement of covering power is disclosed in U.S. Pat. Nos. 4,434,226, 4,414,310, 4,433,048, 4,414,306, and 4,459,353.
Also, multilayer color photographic materials using a tabular grain silver halide emulsion having an aspect ratio of at least 8:1 for the high-speed emulsion layer and having a high sensitivity, improved graininess, sharpness and color reproducibility are disclosed in Japanese Patent Application (OPI) Nos. 113930/83, 113934/83, and 119350/84 (the term "OPI" as used herein means an "unexamined published patent application").
The above-describedpatents (applications) describe that when a tabular grain silver halide emulsion is used for a blue-sensitive elulsion layer of a multilayer color photographic material, the sharpness is improved since the tabular grain has less light-scattering property and when a tabular grain emulsion is used for a green- or red-sensitive layer, the granularity is also improved.
Japanese Patent Application (OPI) No. 77847/86 discloses a multilayer color photographic material having improved sharpness and color reproducibility by using a tabular grain silver halide emulsion having an aspect ratio of at least 5 for a high-speed emulsion layer and a monodisperse silver halide emulsion for a low-speed layer.
Still further, Research Disclosure, No. 25330 discloses a method of controlling the thickness of tabular grains so that the amount of light (sensitive to a silver halide emulsion in an emulsion layer (upper layer) above the emulsion layer using the tabular grains) reflected from the tabular grain emulsion layer is increased to increase the sensitivity of the upper layer or so that the amount of the reflected light is minimized to keep the sharpness of the upper layer.
It has now been found by the inventors' experiments that in spite of such various merits of the tabular grain silver halide emulsion having a high aspect ratio, when in the multilayer structure most generally used for a multilayer color photographic material (i.e., a multilayer structure of a red-sensitive layer, a green-sensitive layer, and a blue-sensitive layer, disposed in this order from the support side), the tabular grain silver halide emulsion having a high aspect ratio (e.g., an aspect ratio of at least 8) is used as a layer other than the layer farthest from the support, in particular, as a green- or red-sensitive layer, the sharpness of the low frequency side is reduced.
For overcoming the reduction in sharpness of the green- or red-sensitive emulsion layer, a silver halide color photographic material containing a tabular grain silver halide emulsion having am aspect ratio of at least 5 in at least one blue-sensitive emulsion layer and also substantially monodisperse non-tabular grains having an aspect ratio of less than 5 in at least one green-sensitive emulsion layer or red-sensitive emulsion layer is proposed in Japanese Patent Application No. 235763/86.
In the proposition, it is said that normal crystal silver halide grains such as tetradecahedral grains are particularly preferred as such a monodisperse non-tabular grain but by the inventors' experiments, it has further been found that the use of such normal crystal grains is yet insufficient with respect to sensitivity and granularity.
This is believed to be due to the fact that although the normal crystal grains have uniform grain sizes and have good granularity as to mean grain size (i.e., as compared to other crystal form grains of the same grain size), the sensitivity as to mean grain size is low.
On the other hand, it has been suggested for a long time that since in tabular grains which are twin grains, and in particular multiple parallel twin grains having a concave incident angle, the concave incident angle is disposed at a position for readily forming a chemical sensitization center, these tabular grains prevent the dispersion of latent images and have high sensitivity as compared to normal crystal grains.
From such a view point, the inventors have started to develop tabular grains which have uniform features such as form, size, etc., like the aforesaid normal crystal grains, have a good relation of sensitivity and granularity, and do not reduce the sharpness at the low frequency side even when they are used for a green- or red-sensitive emulsion layer.
Japanese Patent Application (OPI) Nos. 39027/76, 153428/77, and 142329/80 disclose a production process for multiple parallel twin grains having a monodisperse concave incident angle, but as the result of the inventors' investigations, it has been confirmed that even with these multiple twin crystal grains, a sufficient granularity and sensitivity is not obtained.
By the detailed determination of the spacing between the twin planes of these grains by use of an electron microscope, an X-ray microanalyzer, or the method described hereinafter, it has been found that many defects exist in the grains, the iodine distribution in the grains is broad, and the thickness (b) of the grains and b/a varies in grains, wherein a means the longest distance between two or more parallel twinning planes of a silver halide crystal grain.
This shows that in twin grains which are monodisperse grains in appearance, the features of each grain are not uniform among the grains. Accordingly, the inventors have considered that for obtaining a good relation of granularity and sensitivity, it is necessary that the above-described features of each grain be uniform among the grains.